PCSX2 Documentation/Nightmare on Floating-Point Street: Difference between revisions
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PCSX2 Documentation/Nightmare on Floating-Point Street (view source)
Revision as of 19:42, 19 July 2015
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(Created page with "It is very hard to emulate the floating-point calculations of the R5900 FPU and the Vector Units on an x86 CPU because the Playstation 2 does not follow the IEEE standard. Mul...") |
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''Originally written by ZeroFrog'' | |||
It is very hard to emulate the floating-point calculations of the R5900 FPU and the Vector Units on an x86 CPU because the Playstation 2 does not follow the IEEE standard. Multiplying two numbers on the FPU, VU, and an x86 processor can give you 3 different results all differing by a couple of bits! Operations like square root and division are even more imprecise. | It is very hard to emulate the floating-point calculations of the R5900 FPU and the Vector Units on an x86 CPU because the Playstation 2 does not follow the IEEE standard. Multiplying two numbers on the FPU, VU, and an x86 processor can give you 3 different results all differing by a couple of bits! Operations like square root and division are even more imprecise. | ||
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In the end Pcsx2 does all its floating-point operations with SSE since it is easier to cache the registers. Two different rounding modes are used for the FPU and VUs. Whenever a divide or rsqrt occur on the FPU, overflow is checked. Overflow is checked much more frequently with the VUs. The fact that VUs handle both integer and floating-point data in the same SSE register makes the checking a little longer. In the future, Pcsx2 will read the rounding mode and overflow settings from the patch files. This is so that all games can be accommodated with the best/fastest settings. | In the end Pcsx2 does all its floating-point operations with SSE since it is easier to cache the registers. Two different rounding modes are used for the FPU and VUs. Whenever a divide or rsqrt occur on the FPU, overflow is checked. Overflow is checked much more frequently with the VUs. The fact that VUs handle both integer and floating-point data in the same SSE register makes the checking a little longer. In the future, Pcsx2 will read the rounding mode and overflow settings from the patch files. This is so that all games can be accommodated with the best/fastest settings. | ||
Moral of the blog When comparing two floating point numbers a and b, never use a == b. Instead use something along the lines of | '''Moral of the blog''' | ||
When comparing two floating point numbers a and b, never use <code>a == b</code>. Instead use something along the lines of <code>fabs(a-b)< epsilon</code> where epsilon is some very small number. | |||
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